adversary

An adversary is someone who fights against or opposes another. In tennis, you stand across the net from your adversary.

Adversary as in "enemy" or "opponent" is related to the words adversarial, or hostile, or adverse, meaning against or contrary. The Adversary is a specific reference to Satan, or the Devil. The word adversary is from Middle English adversarie, from Latin adversarius, from adversus "against."